When transmitting data from GERAN to user equipment and vice versa, the data to be transmitted must be encrypted (ciphered) before transmission for security reasons. Encryption makes tapping of signaling and user data difficult. The data to be transmitted is encrypted using an encryption algorithm at the transmitting end and the encrypted data is transmitted from the transmitting end to the receiving end, where the transmitted data is decrypted using an encryption algorithm. The same encryption algorithm is used at both ends.
An encryption mask created by the encryption algorithm is attached to the data to be encrypted using an XOR operation (logical exclusive OR operation), so the encryption does not per se increase the number of bits to be transmitted. This can be presented by the formulaC=M⊕P  (1)wherein C is the encrypted data, M is the encryption mask, P is the unencrypted data and ⊕ is the XOR operation.
The encryption algorithm requires input parameters to make the encryption mask created by the algorithm different for each user and each usage time. The most important parameter is the encryption key whose length is 128 bits, for instance. A different encryption key, and thus also a different encryption mask, is used for each user. However, a problem arises from the fact that the same encryption mask cannot be used twice for data with different content. This prohibited situation can be described by the formula
                                                              ⁢                                                    P                1                            ⊕              M                        =                                                            C                  1                                ⁢                                                                  ⊕                                                                  ⁢                                  P                  2                                ⊕                M                            =                              C                2                                                                                        P              1                        ⊕                          P              2                                =                                    C              1                        ⊕                          C              2                                                          (        2        )            
wherein P1 and P2 are unencrypted data with different content and C1 and C2 are encrypted data with different content. As can be seen, a possible eavesdropper can remove the mask by performing an XOR operation between the data having different content and encrypted using the same mask, thus breaking the encryption.
Because of this, other parameters are also used in encryption algorithms, for instance the encryption algorithm of a radio access network (UTRAN) employing the wideband code division multiple access method of the universal mobile telecommunications system (UMTS) uses as input parameters a counter parameter which changes with time, a directional parameter (uplink/downlink) and a bearer parameter.
The structure of the encryption algorithm to be used in GERAN has not yet been decided. It should, however, meet at least the following requirements:                implicit encryption synchronization, especially in connection with handover,        similar approach to both real-time and non-real-time services,        incremental redundance,        multiplexing several different users to the same time slot,        multiplexing several different radio bearers to the same user equipment,        enabling multi-slot operation.        